There is an increased need to reduce levels of organic contaminants in treated industrial wastewater and drinking water. Increased need for treatment of industrial wastewater discharge is the result of the National Pollution Discharge Elimination System (NPDES) which was established as a result of the Clean Water Act of 1972. NPDES controls discharges from point sources of water pollution, and focuses its efforts on monitoring, enforcing and permitting industries that produce waste water. Two organic contaminant classes regulated by NPDES are contaminants leading to biochemical oxygen demand (BOD), and fats, oils and greases (FOG). Both classes of contaminants can be toxic to humans and the environment, and interfere with biologic water treatment systems. Examples of sources of FOG and BOD are petroleum and chemical manufacturing, metal-finishing, food processing, paper-making and textiles.
Additionally, new rules proposed by the Environmental Protection Agency, under the Safe Drinking Water Act, can lead to increased monitoring and regulation of trace organic contaminants in drinking water. Drinking-water purification plants throughout the United States and Europe utilize chlorine as a primary means of disinfection. However, the chlorine reacts with organic contaminants to produce small but significant amounts of compounds such as methylene chloride, methyl chloride, methyl bromide, bromoform, and dichlorobromomethane, collectively called trihalomethanes (THMs). Most THMs are carcenogenic and their concentrations in municipal drinking waters must be closely monitored.
Other organic contaminants in drinking water may be toxic to living organisms, or impart unwanted characteristics to water like taste, odor, color or turbidity. In addition, contaminants may be biologically active pathogenic microorganisms, such as Giardia cysts and Cryptosporidium. Sources of contaminants can be, for example, petroleum from rainwater run-off or leaf-debris or other natural decaying organic matter.
Organic contaminants of this type are generally in the size ranges of about 10.sup.-7 to 10.sup.-2 cm in diameter and are too small to be removed by physical filtration. They also resist settling out by gravity. They can include a wide range of organic debris with different molecular weights, and abilities to form dispersion, polar, hydrogen bonding or other inter-molecular interactions. Some can dissolve in water completely, others can show partial solubilities due to these polar or hydrogen interactions. In addition, there can also be significant amounts of surface-active molecular debris that contain hydrophobic and hydrophilic components. This debris tends to absorb onto other contaminants and impart to them slight but significant polar characteristics, promoting formation of meta-stable colloidal suspensions.
The most economical treatment method for removing these contaminants from drinking and waste water is typically chemical destabilization followed by gravity settling, filtration or dissolved air flotation. Because destabilization chemistry depends on the interactions between polar functional groups contained on the contaminants to achieve coagulation and flocculation, those that do not carry polar groups are not appreciably removed by this chemistry. Additionally, destabilization chemistry can only remove contaminants that are not dissolved in water. These two factors leave the majority of BOD, FOG, and organic contaminants currently outside of the reach of this treatment method.